1. Technical Field of the Invention
The present invention relates generally to a gas concentration measuring device which may be employed in an air-fuel ratio control system for automotive vehicles to measure a given gas component contained in emissions from an internal combustion engine, more particularly to an improvement on a circuit structure of a gas concentration measuring device equipped with a gas sensor which, when applied with the voltage, produces an electric signal indicative of the concentration of gas.
2. Background Art
Recently, in order to meet requirements for improvement in control accuracy of automotive air-fuel ratio control systems and enhancement of lean burn of internal combustion engines, linear air-fuel ratio sensors designed to measure the concentration of oxygen contained in exhaust gasses of the internal combustion engine to determine the air-fuel ratio of mixture sucked into the engine linearly in a wide range and an air-fuel ratio measuring devices using the same have been proposed. As such air-fuel ratio sensors, a limiting current air-fuel ratio sensor as taught in, for example, U.S. Pat. No. 5,691,464 is known in the art which is responsive to application of voltage to produce a limiting current whose detectable range changes with a change in concentration of oxygen in exhaust gasses.
FIG. 1 shows an air-fuel ratio measuring circuit 80 employed in one example of conventional air-fuel ratio measuring devices.
The air-fuel ratio measuring circuit 80 includes a reference voltage generator 84, amplifying circuits 85 and 86, a current-detecting resistor 88, and a voltage follower 89.
The reference voltage-generator 84 produces a constant reference voltage Va. The reference voltage Va is amplified in current by an operational amplifier 85a of the amplifying circuit 85. To one end of an air-fuel ratio sensor 81, the voltage identical with the reference voltage Va is applied. An operational amplifier 86a of the amplifying circuit 86 amplifies in current a command voltage Vb produced from a D/A converter 87. The voltage equal to the command voltage Vb is applied to the other end of the air-fuel ratio sensor 81. The command voltage Vb is adjusted by a CPU (not shown) according to an instantaneous air-fuel (A/F) ratio.
The sensor current flows through the air-fuel ratio sensor 81 as a function of the A/F ratio of gases to be measured. A voltage drop across the resistor 88 caused by the flow of the sensor current, that is, a difference between the reference voltage Va and the voltage Vc is monitored by an external electronic control unit (ECU) to determine the value of the A/F ratio. The voltage Vc is inputted to the ECU through the voltage follower 89. The value of the A/F ratio determined in the ECU is employed in the feedback control of the A/F ratio.
FIG. 2 shows a typical circuit structure of each of the operational amplifiers 85a and 86a. The operational amplifiers 85a and 86a have the same circuit structure, and explanation below will refer only to the operational amplifier 85a for the brevity of disclosure.
The operational amplifier 85a operates on a source voltage Vcc of 5 V. An input circuit 91 includes a pair of pnp transistors T21 and T22 which operate on the constant current I1 from a constant current circuit C1 in response to input signals IN+ and INxe2x88x92 to change the collector current as a function of a difference in voltage between the input signals IN+ and INxe2x88x92. Changes in collector current of the transistors T21 and T22 will activate a pair of npn transistors T23 and T24.
Specifically, when the input signal IN+ is higher in voltage than the input signal INxe2x88x92, it will cause the collector current of the pnp transistor T22 to increase, so that the collector voltage of the npn transistor T24 is elevated. Alternatively, when the input signal IN+ is lower in voltage than the input signal INxe2x88x92, it will cause the collector current of the pnp transistor T21 to increase, so that the base current flows in the npn transistors T23 and T24, thereby turning on the npn transistors T23 and T24 so that the collector voltage of the transistor T24 drops.
The collector voltage of the npn transistor T24 is transferred as a signal SG1 to the intermediate amplifying circuit 92. The signal SG1 is amplified and outputted as a signal SG2 to the bias circuit 93. The bias circuit 93 operates on the constant current I2 from the constant current circuit C2 and activates the npn transistor T25 working as a current source or the npn transistor T26 working as a current sink.
When the input signal IN+ is higher in voltage than the input signal INxe2x88x92, the bias circuit 93 activates the npn transistor T25 to elevate an output voltage. Alternatively, when the input signal N+ is lower in voltage than the input signal INxe2x88x92, the bias circuit 93 activates the npn transistor T26 to decrease the output voltage.
Each of the operational amplifiers 85a and 86a, however, has the drawback in that a voltage output is produced only within a range narrower than a range from the source voltage Vcc to ground potential by given voltage losses. Increasing the accuracy in measuring the concentration of gas requires broadening the range of the output voltage.
The reason that the range of the output voltage is limited to be narrower than the range from the source voltage Vcc to ground potential will be discussed below.
The voltage of the input signal IN+ depends upon a voltage drop VI1 across the constant current circuit C1 and the base-emitter voltage VF1 of the transistor T21 (or the base-emitter voltage VF2 of the transistor T22). Specifically, the voltage of the input signal INxe2x88x92 depends upon the voltage drop VI1 and the base-emitter voltage VF2 developed across the transistor T22. The transistors T21 and T22, therefore operate normally within a voltage range below Vcc-VI1-VF1 (or -VF2). If VF1=VF2=0.7 V and VI1=0.6 V, then a maximum voltage of each of the input signals IN+ and INxe2x88x92 is restricted to 5 Vxe2x88x920.6 Vxe2x88x920.7 V=3.7 V.
The npn transistor T25 operates on the constant current I2 from the constant current circuit C2 and allow the base current to flow. An upper limit of the output voltage of the transistor T25, thus, depends upon the voltage drop VI2 developed across the constant current circuit C2 and the base-emitter voltage VF5 developed across the transistor T25. Specifically, the upper limit of the output voltage of the transistor is limited to below Vccxe2x88x92VI2xe2x88x92VF5. If VF5=0.7 V and VI2=0.6 V, then a maximum output voltage will be 5 Vxe2x88x920.6 Vxe2x88x920.7 V=3.7 V.
The pnp transistor T26 is turned on, causing the base current to flow into the bias circuit 93. If the base-emitter voltage VF6 of the transistor T26 is 0.7 V, then a lower limit of the output voltage of the transistor T26 is restricted to VF6=0.7 V where a voltage drop of the bias circuit 93 is assumed to be zero (0).
Therefore, the voltage of output from each of the operational amplifiers 85a and 86a falls within a range of 0.7 to 3.7 V which is narrower than a source voltage-to-ground potential range of 0 to 5 V.
Additionally, when an air-fuel ratio of 25 is measured in a lean. burn range of the engine, the sensor current flowing through the A/F sensor 81 shows 22 mA. In this case, the base-emitter voltage VF6 of the pnp transistor T26 increases up to 1.2 V. The output voltage range of each of the operational amplifiers 85a and 86a will, thus, be decreased to 1.2 to 3.7 V. Note that if VF6=0.7 V as described above, then the sensor current=1 mA in a rich burn range of the engine.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to increase an input/output voltage range of an operational amplifier used in a gas concentration measuring device for improving the accuracy in measuring the concentration of gas.
According to one aspect of the invention, there is provided a gas concentration measuring apparatus which comprises a gas concentration sensor exposed to a gas and a voltage applying circuit. The gas concentration sensor is responsive to application of voltage to produce a current signal indicative of concentration of the gas. The voltage applying circuit includes an operational amplifier which operates on a source voltage developed between a first and a second source terminal thereof connected to a voltage source. The operational amplifier outputs voltage for developing the voltage applied to the gas concentration sensor which has a level changing as a function of voltage inputted to the operational amplifier. The operational amplifier is designed to have an amplitude of each of the voltages inputted to and outputted from the operational amplifier which falls within a given input/output voltage range defined between an upper limit and a lower limit of a source voltage range of the voltage developed by the voltage source between the first and second source terminals of the operational amplifier and near at least one of the upper and lower limits of the source voltage range.
In the preferred mode of the invention, a difference between an upper limit of the input/output voltage range and the upper limit of the source voltage range is less than or equal to 0.6 V.
A difference between a lower limit of the input/output voltage range and the lower limit of the source voltage range may also be less than or equal to 0.6 V.
The gas concentration sensor measures the concentration of a preselected component of exhaust gasses of an engine mounted in an automotive vehicle. The first source terminal of the operational amplifier is connected to a positive terminal of a single battery installed in the vehicle, while the second source terminal is kept at a reference potential.
The voltage source may alternatively be a constant voltage source for a digital signal connecting with the first source terminal of the operational amplifier. The upper limit of the input/output voltage range lies between the voltage provided by the constant voltage source and the voltage provided by the constant voltage source minus 0.6 V, while the lower limit of the input/output voltage range lies between a ground potential and the ground potential plus 0.6 V.
The operational amplifier has an npn transistor disposed in an output stage thereof. The npn transistor serves as a current sink element and connects at an emitter with ground and at a collector with an output terminal of the operational amplifier.
The operational amplifier also has a pnp transistor disposed in the output stage thereof. The pnp transistor serves as a current source element and connects at an emitter with the voltage source and at a collector with an output terminal of the operational amplifier.
The operational amplifier includes a first input stage to which a higher voltage is inputted and a second input stage to which a lower voltage is inputted.
The operational amplifier is designed to have a rail-to-rail structure.
A resistor circuit, a voltage signal outputting circuit, and a resistor changing circuit are further provided. The resistor circuit is disposed between the output terminal of the operational amplifier and the gas concentration sensor. The voltage signal outputting circuit outputs voltage appearing across the resistor circuit changing as a function of the current signal flowing through the gas concentration sensor. The resistor changing circuit changes a resistor value of the resistor circuit as a function of a value of the current signal.
The resistor changing circuit decreases the resistor value of the resistor circuit as the concentration of the gas increases.
According to another aspect of the invention, there is provided a gas concentration measuring apparatus which comprises a gas concentration sensor exposed to a gas and a first and a second operational amplifier. The gas concentration sensor produces a current signal indicative of concentration of the gas when input voltage is developed across a first and a second terminal of the gas concentration sensor. The first operational amplifier operates on a source voltage developed between a first and a second source terminal thereof connected to a voltage source and outputs voltage to develop a first electric potential at the first terminal of the gas concentration sensor. The voltage outputted from the first operational amplifier changes as a function of voltage inputted to the first operational amplifier. The second operational amplifier operates on the source voltage developed between a first and a second source terminal thereof connected to the voltage source and outputs voltage to develop a second electric potential at the second terminal of the gas concentration sensor for creating the input voltage applied to the gas concentration sensor. The voltage outputted from the second operational amplifier changes as a function of voltage inputted to the second operational amplifier. Each of the first and second operational amplifiers is designed to have an amplitude of each of the voltages inputted thereto and outputted therefrom which falls within a given input/output voltage range defined between an upper limit and a lower limit of a source voltage range of the voltage developed by the voltage source between the first and second source terminals of one of the first and second operational amplifiers and near at least one of the upper and lower limits of the source voltage range.
In the preferred mode of the invention, a resistor circuit and a voltage signal outputting circuit are further provided. The resistor circuit is disposed between an output terminal of the first operational amplifier and the gas concentration sensor. The voltage signal outputting circuit outputs voltage appearing across the resistor circuit changing as a function of the current signal flowing through the gas concentration sensor.
A resistor changing circuit is further provided which changes a resistor value of the resistor circuit as a function of a value of the current signal.